Method for the surface treatment of corten steel

ABSTRACT

The present invention relates to a method for the surface treatment of Corten steel, in particular comprising the steps of cleaning, activating, coloring, and sealing the surface. The invention further relates to Corten steels produced by the method according to the invention.

This application is the U.S. national phase of International Application PCT/EP2014/062416, filed Jun. 13, 2014, claiming priority to German Patent Application 102013010080.1 filed Jun. 14, 2013; the disclosures of International Application PCT/EP2014/062416, and German Patent Application 102013010080.1, each as filed, are incorporated herein by reference.

The present invention relates to a method for producing surface-treated Corten steel, comprising the steps of cleaning, activating, coloring, and sealing the surface. The invention further relates to Corten steels produced by the method according to the invention.

PRIOR ART

Corten steels (for example, Material Nos. 1.8946 ASTM A 242 and 1.8963 ASTM A 588 according to ASTM A 242) form a rust layer on their surface during weathering, which at its base is particularly seal-tight and prevents or at least greatly slows the development of corrosion. The surfaces then have a reddish-brown rust layer.

Due to the relatively good corrosion resistance paired with favorable acquisition costs, Corten steels are attractive for use in architecture, for example as facade cladding. However, they have several disadvantages, which thus far have precluded widespread use despite favorable costs.

The corrosion process at the surface does not come to a complete halt, in particular in a corrosive atmosphere, which is usually present in cities and industrial areas. The rust layer on Corten steels is then porous on its outer surface and mechanically unstable. During contact or in flowing water it releases rust particles, which may result in rust stains on the hands or clothing.

Numerous attempts are known from the literature for stabilizing the rust layer on Corten steels by applying lacquer layers or by impregnating with waxes. All of these attempts have failed due to the fact that lacquer layers do not adhere to the unstable base surface, but instead peel off, and waxes themselves are not permanently weather-resistant.

Another disadvantage is that a colored design other than rust brown is not possible. Such surfaces do not have a very high visual appeal, and the options for achieving architectonic accents by coloring are very limited.

One significant drawback in the use of Corten steel in architecture is that the patina forms slowly over time, and it may take years until the final state is reached and the building has the desired appearance.

There are numerous formulations for pretreating new Corten steel in order to rapidly produce a rust layer having an appearance that is similar to the end state. These formulations are usually based on aqueous solutions with very corrosive acid mixtures, which essentially contain hydrochloric acid and a number of other acids, such as sulfuric acid or nitric acid. The major drawback of these pretreatments is that the rust layers that are obtained do not correspond to those which result from natural aging, and may contain residues of the corrosive media from the pretreatment which, due to the porosity of the layers, cannot be flushed out free of residue. As a result, these substances, for example chlorides, promote corrosion and may hinder retardation of the subsequent corrosion along with the formation of a protective patina.

In the field of architecture, there is great interest in being able to use cost-effective Corten steels whose surfaces may be designed in colors other than drab rust brown, which are stable against weathering and abrasion, and which do not cause rust stains. The present invention fulfills these requirements. Therefore, it is an object of the present invention to provide novel Corten steels and a method for producing same, wherein these Corten steels have the above-mentioned advantages and/or overcome the above-mentioned disadvantages.

SUMMARY OF THE INVENTION

The subject matter of the present invention relates to a method, as defined in the claims, for producing various color effects in a controlled and targeted manner by means of a wet chemical treatment of surfaces on Corten steels, and permanently protecting the colored surfaces against further corrosion and change, and soiling.

The present invention therefore relates to a method for producing surface-treated Corten steel, comprising the following steps:

Step (i): providing Corten steel and carrying out a treatment, including or comprising the following stages:

Stage (1): cleaning at least a portion of at least one surface of the Corten steel;

Stage (2): activating at least a portion of the cleaned surface of the Corten steel from stage (1) by bringing the cleaned surface into contact with an aqueous activating solution containing fluoride ions, one or more oxidizing agents, and optionally one or more stabilizers for the oxidizing agent(s), followed by an optional rinsing step with water in order to remove at least a portion of the aqueous activating solution from the activated surface; and

Stage (3): coloring at least a portion of the activated surface of the Corten steel from stage (2) by treating the activated surface with water for a period of at least 30 minutes and until the desired coloration of the surface of the Corten steel is achieved; carrying out an optional rinsing step with water if aqueous activating solution is still present on the colored surface; and carrying out a drying step; and

Step (ii): sealing at least a portion of the colored surface from stage (3) of step (i) by applying a colorless, transparent coating, as the result of which the surface-treated Corten steel is obtained.

The invention further relates to a Corten steel having a treated surface, containing a colorless, transparent coating on a colored surface of the Corten steel, wherein the surface treatment is carried out by a method according to the present invention.

DESCRIPTION OF THE INVENTION

The method in question involves a treatment of the surface of Corten steel in two steps. In step (i), Corten steel is provided, and a desired color is produced by wet chemical treatment. Step (ii) comprises sealing of the colored surface in order to permanently protect it from further corrosion and change, and to provide it with dirt-repellent properties.

The present invention therefore relates to a method for producing surface-treated Corten steel, including or comprising the following steps:

(i): providing Corten steel and carrying out a treatment including or comprising the following stages:

(1): cleaning at least a portion of at least one surface of the Corten steel;

(2): activating at least a portion of the cleaned surface of the Corten steel from stage (1) by bringing the cleaned surface into contact with an aqueous activating solution containing fluoride ions, the fluoride ions originating, for example, from hydrofluoric acid, ammonium bifluoride, sodium bifluoride, potassium bifluoride, in particular from hydrogen fluoride (HF); one or more oxidizing agents, and optionally one or more stabilizers for the oxidizing agent(s), followed by an optional rinsing step with water in order to remove at least a portion of the aqueous activating solution from the activated surface; if no smoothing of the surface has to take place, HCl, H₂SO₄, or FeCl₃ may be used, and

(3): coloring at least a portion of the activated surface of the Corten steel from stage (2) by treating the activated surface with water for a period of at least 30 minutes and until the desired coloration of the surface of the Corten steel is achieved; carrying out an optional rinsing step with water if aqueous activating solution is still present on the colored surface; and carrying out a drying step; and

(ii): sealing at least a portion of the colored surface from stage (3) of step (i) by applying a colorless, transparent coating, as the result of which the surface-treated Corten steel is obtained.

Step (i)

Step (i) comprises the provision of Corten steel and a treatment comprising the three stages of (1) cleaning, (2) activating, and (3) coloring.

Corten steel is initially provided. For example, a Corten steel according to elemental analysis may include or comprise: iron as the main component (for example, greater than 90% by weight or greater than 95% by weight), and one or more components selected from the group comprising: carbon in a proportion of 0 to 0.16% by weight, chromium in a proportion of 0.40 to 0.65% by weight, silicon in a proportion of 0.30 to 0.50% by weight, manganese in a proportion of 0.80 to 1.25% by weight, phosphorus in a proportion of 0 to 0.030% by weight, sulfur in a proportion of 0 to 0.030% by weight, copper in a proportion of 0.10 to 0.40% by weight, vanadium in a proportion of 0.02 to 0.10% by weight, and nickel in a proportion of 0 to 0.40% by weight. The above percentages refer to the total mass of all components in the Corten steel.

In one embodiment, the Corten steel is composed of iron in a proportion of at least 90% by weight or at least 95% by weight, and the following components: carbon in a proportion of 0 to 0.16% by weight, chromium in a proportion of 0 to 0.65% by weight, silicon in a proportion of 0 to 0.50% by weight, manganese in a proportion of 0 to 1.25% by weight, phosphorus in a proportion of 0 to 0.030% by weight, sulfur in a proportion of 0 to 0.030% by weight, copper in a proportion of 0 to 0.40% by weight, vanadium in a proportion of 0 to 0.10% by weight, and nickel in a proportion of 0 to 0.40% by weight.

Examples of Corten steels are Material Nos. 1.8946 and 1.8963 according to ASTM A 242 and A 588.

The Corten steel may be present as a material/starting material, for example as sheet steel, or as a product, for example as a component of a finished structure. The Corten steel should not be treated or coated. However, the surface must be clean.

The provided Corten steel is then cleaned. The cleaning in stage (1) of step (i) takes place in such a way that the cleaned surface of the Corten steel is metallically clean, free of grease, and free of rust.

The cleaning in stage (1) of step (i) may take place in such a way that the surface of the Corten steel is first subjected to alkaline hot degreasing (for example, using the AK 161 from Schlötter), followed by intermediate rinsing of the surface with water, followed by a treatment with a pickling inhibitor (for example, BESTA-S from Poligrat, Germany, containing sulfuric acid amidosulfonic acid, oxalic acid, and inhibitors, or a pickling inhibitor containing sulfuric acid, orthophosphoric acid, dimethylsulfoxide, and hexamethyltetramine, for example), likewise followed by a rinse treatment in municipal water/drinking water/tap water.

The second stage, stage (2) in step (i), is used for activating the cleaned surface or the cleaned surfaces for the subsequent coloring. The activation is carried out by treating the cleaned surface with an aqueous activating solution (solution A) by dipping and/or spraying. The treatment time at room temperature (20° C.) may be, for example, 2 to 30 minutes, or 2 to 15 minutes, preferably 2 to 5 minutes.

The aqueous activating solution may be prepared using drinking water/process water/tap water by adding HF and oxidizing agent as well as one or more optional stabilizers for the oxidizing agent. The aqueous activating solution preferably contains no acids stronger than HF, based on the pKa value. The aqueous activating solution preferably contains HF as the only acid.

The aqueous activating solution is preferably composed of HF, oxidizing agent(s), optionally one or more stabilizers for the oxidizing agent(s), and drinking water/process water/tap water.

The aqueous activating solution may contain fluoride ions, for example by using hydrogen fluoride, in a quantity of greater than 0% by weight and less than or equal to 3% by weight, or greater than 0.5% by weight and less than or equal to 3% by weight, preferably greater than 0.5% by weight and less than or equal to 1% by weight, based on the weight of the aqueous activating solution. A solution containing such a quantity of fluoride is advantageously not classified as toxic. The active fluoride portion is determined based on the material removal from a test sheet. The active fluoride content and the material removal are linearly related.

At least a portion of the quantity or also the entire quantity of fluoride ions preferably originates from hydrogen fluoride which has been added to the aqueous activating solution. The fluoride ions may also originate from other sources, for example ammonium bifluoride, sodium bifluoride, and/or potassium bifluoride.

A suitable, preferred oxidizing agent is hydrogen peroxide. Hydrogen peroxide may be used as an aqueous solution. The quantity of oxidizing agent may be selected by those skilled in the art depending on the oxidizing agent used, for example by conducting simple test trials.

The aqueous activating solution may contain, for example, the oxidizing agent(s), preferably hydrogen peroxide, in a quantity of greater than 0% by weight and less than or equal to 30% by weight, or greater than 0% by weight and less than or equal to 20% by weight, preferably greater than 3% by weight and less than or equal to 20% by weight, or greater than 3% by weight and less than or equal to 10% by weight.

The aqueous activating solution may in particular contain fluoride ions, for example from hydrogen fluoride, in a quantity of greater than 0.5% by weight and less than or equal to 3% by weight, preferably greater than 0.5% by weight and less than or equal to 1% by weight, based on the weight of the aqueous activating solution, and may contain the oxidizing agent(s), preferably hydrogen peroxide, in a quantity of greater than 3% by weight and less than or equal to 20% by weight, based on the weight of the aqueous activating solution.

Examples of suitable optional stabilizers for the oxidizing agent are mixtures of urea and one or more alkane diphosphonic acids which are optionally substituted with one or more hydroxyl or amino groups, or the salts thereof. The hydrocarbon chain of the alkane diphosphonic acids preferably contains 1, 2, 3, or 4 C atoms. Examples of such alkane diphosphonic acids are alkylene diphosphonic acids or amino- or hydroxy-substituted alkylidene diphosphonic acids. 1-Hydroxyethane-1,1-diphosphonic acid is a particularly suitable alkylidene diphosphonic acid. Suitable stabilizers are also described in EP 1 903 081.

The quantity of stabilizers in the aqueous activating solution may be selected as described in EP 1 903 081. Particularly high effectiveness of this stabilizer has been observed when the weight ratio of urea to free alkane diphosphonic acids is in the range of 100:1 to 20:1. This weight ratio is preferably between 60:1 and 35:1, in particular approximately 50:1.

The stabilizer is used for stabilizing aqueous peroxide-containing solutions. These are primarily understood to mean aqueous hydrogen peroxide-containing solutions, although other peroxide-containing solutions may also be stabilized in this way, for example solutions containing persulfuric acids and/or peroxycarboxylic acids, such as peracetic acid, or the salts thereof.

Chlorides/phosphates should not be present.

The concentration of a stabilized aqueous solution of hydrogen peroxide may be 30% or 35%, for example, but may also be lower, for example 20%, 10%, or 5%.

For example, solution A may contain fluorides in a maximum quantity of 1% by weight, 0.8% hydrofluoric acid, and a strongly oxidizing component, for example hydrogen peroxide, in a quantity of approximately 5% by weight, together with a suitable stabilizer (for example, according to EP 1903081 (A2) from Poligrat).

For example, the activation takes place by treating the cleaned surface in an aqueous solution (solution A) by dipping or spraying. Solution A contains, for example, fluorides in a maximum quantity of 1% by weight HF and a strongly oxidizing component, for example hydrogen peroxide, in a quantity of approximately 5% by weight together with a suitable stabilizer (for example, according to EP 1903081 (A2) or product C600, C410, or C400 from Poligrat). The treatment time at room temperature is approximately 2 to 5 minutes.

The term “municipal water/drinking water/tap water” as used here relates to natural water with the minerals and trace elements typically contained therein.

In one embodiment, the aqueous activating solution contains HF in a quantity of 0.5% by weight to 1% by weight, hydrogen peroxide in a quantity of 2% by weight to 10% by weight, and optionally one or more stabilizers.

After the surface is activated, an optional rinsing step with water may be carried out in order to partially or completely remove the aqueous activating solution. If the aqueous activating solution is not removed, or is only partially removed, this affects the subsequent coloring step. The remaining quantity of the aqueous activating solution may be set depending on the desired color of the surface. The less rinsing carried out, the darker the color.

The actual coloring of the surfaces takes place in the third stage of step (i).

The treatment of the activated surface in stage (3) of step (i) may be carried out using water, for example having a pH in the range of 6-8, over a period of, for example, 30 minutes to 15 hours, preferably 1 hour to 12 hours.

The treatment with water may take place in principle by spraying with water and keeping the treated area moist. In the meantime, the surface should not dry during the treatment. Stage (3) is thus preferably carried out in such a way that the surface does not completely dry during the treatment period. A moist surface could be obtained, for example, by conducting a simple test using a handkerchief, wherein the handkerchief appears moist/wet after contact with the surface. For keeping the area moist, the surface to be treated may, for example, be covered, for example with plastic film, in such a way that the drying of the surface is at least slowed down. The area is preferably kept moist by light spraying or misting with water, similar to spray irrigation in a greenhouse. The quantity of water may be selected by those skilled in the art in such a way that the area to be treated is kept completely and uniformly moist, in particular in order to obtain a uniform color. The more uniform and thin the water film, the more uniform the achieved color. Water runoff produces structured colorations.

Municipal water/drinking water/tap water, for example Munich drinking water, may be used.

There are basically two variants for coloring the surfaces, depending on the desired color.

Variant 1 for producing golden yellow, orange, or light brown surfaces:

After the treatment in solution A, the surfaces are rinsed with water, for example municipal water, rinsed off, and subsequently treated with water, for example municipal water, in a horizontal, tilted, or vertical position, by lightly spraying and keeping the area moist. Depending on the duration of the treatment, various colors then result on the surface of the Corten steel; in particular, the following colors are obtained after the approximate indicated times:

1 hour: gold

3 hours: orange

12 hours: light brown

The method may thus be carried out, for example, in such a way that the optional rinsing step with water takes place in stage (2) of step (i), and the coloring of the activated surface in stage (3) of step (i) is carried out by spraying and keeping the activated surface moist. Preferably very little water is used, for example 0.2-1 L/m² and L/h.

Variant 2 for producing olive green and dark brown surfaces: After the treatment in solution A, the surfaces, for example metal sheets, without rinsing, are sprayed with water, for example municipal water, in a horizontal, tilted, or vertical position, for example, for a period of 1 to 2 minutes, for example, and are subsequently kept only moist. Depending on the duration of the treatment, the following colors result on the surface of the Corten steel; in particular, the following colors are obtained after the approximate indicated times:

3 hours: olive green

12 hours: dark brown

The method may thus be carried out, for example, in such a way that the optional rinsing step with water in stage (2) of step (i) is not carried out, and the coloring of the activated surface in stage (3) of step (i) takes place using little water, in particular only by keeping moist and not by rinsing, for example by spraying once with water and subsequently keeping the area moist by covering the surface, or by lightly spraying with water as described above, continuously or at intervals, to minimize runoff of the activating solution.

Structured color effects, for example stripes, are obtained by treating the surfaces in the first few minutes, for example 10 minutes after the treatment in solution A, in a vertical or tilted position. Depending on the steepness of the surfaces during the treatment and the intensity with which the surfaces are sprinkled or sprayed, different fine or coarse structures result from the original runoff tracks of the water.

After the desired color is achieved, the surfaces are carefully dried and further treated in step (ii).

In step (ii), the colored surfaces are sealed by means of a suitable colorless, transparent coating, preferably a sol-gel coating/sol-gel lacquer (for example, POLIANT or POLISEAL from Poligrat).

The sealing in step (ii) may thus take place, for example, using a sol-gel coating, the sol-gel coating being burned in after the application in order to achieve a glass ceramic structure. However, it is also possible to use other inorganic, transparent, seal-tight, chemically resistant coatings which are able to fully impregnate the colored layer.

It is understood by those skilled in the art that the preferred sol-gel lacquer is initially applied in the form of a liquid sol, with colloidal particles suspended therein, which is subsequently converted into a gel and then forms a solid, hard lacquer layer. Thus, when reference is made to “application of the sol-gel lacquer” or to “hardening of the sol-gel lacquer,” those skilled in the art are aware of which state the sol-gel system is in.

The sol is preferably a silica sol based on silanes dissolved in solvent, for example alcohol. These silica sols may also optionally contain one or more additional sol-forming elements, such as Al, Ti, Zr, Mg, Ca, or Zn, which replace Si atoms in the colloidal structures.

The starting compounds for forming the sols and lastly, the sol-gel lacquer, are hydrolyzable silanes of formula SiR₄, wherein the four R groups contain 2-4 hydrolyzable OR′ groups and 0-2 nonhydrolyzable R″ groups. These starting silanes may also be represented as Si(OR′)_(4-n)R″_(n), where n=0, 1, or 2. If additional sol-forming elements as described above are used, corresponding compounds are to be selected as starting compounds, for example AlR₃, etc., according to the valences of the elements.

The hydrolyzable OR′ groups are hydroxy, alkoxy, and/or cycloalkoxy groups. Suitable examples of such include, for example, hydroxy, methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, isobutoxy, t-butoxy, pentoxy, hexoxy, cyclopentyloxy, and cyclohexyloxy groups, with ethoxy, n-propoxy, and isopropoxy groups in particular being preferred. The hydrolyzable OR′ groups may be the same or different.

The nonhydrolyzable R″ groups, if present, are alkyl and/or cycloalkyl groups. Suitable examples of such include, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, pentyl, hexyl, cyclopentyl, and cyclohexyl groups, with methyl, ethyl, n-propyl, and isopropyl groups in particular being preferred. The nonhydrolyzable R″ groups may likewise be the same or different.

The starting compounds of the sols may be composed of a single type of silane, although they frequently include mixtures of multiple silanes (and optionally additional sol-forming starting compounds of other elements). It is preferred that at least one of the components of the starting compounds is a silane of formula Si(OR′)_(4-n)R″_(n), where n=0, i.e., Si(OR′)₄. For example, a preferred sol-gel lacquer may include the starting materials tetraethoxyorthosilane (TEOS) and methyltriethoxysilane (MTES) and/or dimethyldiethoxysilane (DMDES).

In addition, other additives which are customary in the field of sol-gel systems may of course also be used, for example additional network-forming agents, such as acryloxypropyltrimethoxysilane or methacryloxypropyltrimethoxysilane, which are able to ensure further organic crosslinking, in particular when a considerable portion of the starting compounds are so-called network-converting compounds of formula Si(OR′)_(4-n)R′_(n), where n=1 or 2.

In the sol, the starting compounds are hydrolyzed in part to form the corresponding hydroxy compounds (such as orthosilicic acid, trihydroxyalkylsilane, etc.), which may be facilitated by adding a catalyst, for example acid. Due to the high tendency of these hydroxy compounds toward condensation, they may now condense, with cleavage of water, to form smaller siloxane networks. Colloidal particles containing siloxane bonds are already present in the sol. Siloxane bonds are bonds of the type Si—O—Si, where “≡” symbolizes any three independent bonds with other moieties, in particular with OH, OR′, and R″, resulting in a three-dimensional crosslinked structure in the colloidal particles. OR′ and R″ have the same meanings as above.

The sol-gel lacquer may be applied in any desired manner, for example by dipping, pouring, spraying, or spreading. However, it is preferably applied by spraying, since this allows precise control of the quantity applied per unit area.

The quantity may be set as needed. For example, a sol-gel lacquer layer may have a layer thickness of up to approximately 6 μm, or approximately 0.5 to 3 μm.

The viscosity of the sol-gel lacquer may be set by those skilled in the art. It is known that, with an appropriately high dilution, the sol has a sufficiently low viscosity in its solvent in order to penetrate into any surface pores that are present.

Suitable solvents for the sol are water and in particular alcohols such as methanol, ethanol, n-propanol, or isopropanol, with ethanol and isopropanol being preferred due to their physical properties and low toxicity of their vapors.

The applied sol may then be reacted to form a gel. This reaction converts the liquid sol into a solid gel layer in which the colloidal particles of the sol crosslink with one another due to further hydrolysis and condensation, and with starting compounds which are not yet hydrolyzed and condensed. This may take place, for example, by evaporating the alcoholic solvent during drying.

After the surfaces are dried, the sol-gel coatings are burned in, thus forming a glass ceramic structure which is firmly adherent, resistant to aging, and insensitive to environmental influences, and which permanently preserves the color effects. Burning in of the coating may be carried out by those skilled in the art, using customary procedures.

For example, the surfaces coated with the gel are subjected to thermal hardening. This takes place at elevated temperatures, whereby the gel is converted into a colorless, transparent, glass-like layer. The silica sol components are converted into even more strongly crosslinked silicon dioxide, which, depending on the composition of the underlying sol, may contain further components such as aluminum oxide, titanium oxide, or zirconium oxide. These layers are hard, closed, and resistant to many of the chemicals with which a surface may come into contact under typical conditions, and are resistant to temperatures up to approximately 500° C.

According to one preferred embodiment, the coated surface is exposed to temperatures of 160° C. to 220° C. during the subsequent hardening of the gel. This hardening should take place for a period of at least 10 minutes, preferably 20 to 45 minutes, for example 30 minutes. The hardening is preferably carried out at temperatures between 180° C. and 210° C., for example 200° C., although temperatures below 180° C. are also suitable for this purpose. In the process, the gel is converted into a hard, colorless, transparent, glass-like lacquer which tightly seals the surface, for its part has no cracks, and imparts the surface with a high degree of hardness and wear resistance.

The processes of gel formation and hardening of the gel may be combined, since, for example, gel formation by drying and evaporation of the solvent may also take place, at least partially, at the beginning of the hardening treatment. In addition, such a method in which the processes of gel formation and hardening of the gel are combined is encompassed by the invention.

However, other conventional hardening processes, in particular cathodic hardening, may also be carried out. Cathodic hardening may take place according to patent application DE 21 26 129, for example.

The invention further relates to a Corten steel having a treated surface containing a colorless, transparent coating on a colored surface of the Corten steel, wherein the surface treatment is carried out by a method according to the present invention. The Corten steel according to the invention differs structurally from known Corten steels, which is discernible based on the color and the properties, such as stability against corrosion.

EXAMPLES Example 1

A metal sheet made of material No. 1.8963 having dimensions of 500×500×1.5 mm was degreased in an aqueous hot degreaser (AK 161 from Schlötter) at 60° C. for a period of 10 minutes.

The surfaces were subsequently rinsed twice with municipal water and then derusted in a pickling inhibitor (BESTA-S from Poligrat, Germany) at 60° C. for a period of 2 minutes.

The surfaces were subsequently rinsed twice with municipal water and then activated at room temperature (20° C.) by dipping in an aqueous solution containing 0.8% by weight HF, 5% by weight hydrogen peroxide, and a stabilizer for the peroxide (product C600 from Poligrat) for a period of 4 minutes.

The surfaces were subsequently rinsed once with municipal water and then set down. In the horizontal position, the surfaces were kept moist by lightly spraying with municipal water for a period of 3 hours at room temperature. An orange-colored surface resulted.

The surface was dried and subsequently coated with the POLIANT sol-gel method from Poligrat, using a spray process, and burned in at 200° C. for a period of 30 minutes.

The surface obtained was orange-colored, having an attractive striped structure of lighter and darker colors, with a metallic effect that reflected the light iridescently. The surface was hydrophobic, so that water rolled off in beads without leaving behind traces of moisture.

Example 2

A metal sheet corresponding to Example 1 was pretreated and activated as in Example 1. The metal sheet, without a rinsing process, was subsequently kept moist at room temperature in a horizontal position by spraying with municipal water. After a period of 3 hours and subsequent rinsing, the metal sheet had an olive green surface. Further treatment was carried out as in Example 1, with the metallic effect on the surface being slightly less pronounced in comparison to Example 1. 

1. A method for the surface treatment of Corten steel, comprising the following steps: (i) providing Corten steel and carrying out a treatment comprising the following stages: (1) cleaning at least a portion of at least one surface of the Corten steel, thereby forming a cleaned surface; (2) activating at least a portion of the cleaned surface of the Corten steel from stage (1) by bringing the cleaned surface into contact with an aqueous activating solution containing fluoride ions, one or more oxidizing agents, and optionally one or more stabilizers for the oxidizing agent(s), thereby forming an activated surface; and (3) coloring at least a portion of the activated surface of the Corten steel from stage (2) by treating the activated surface with water for a period of at least 30 minutes and until the desired coloration of the surface of the Corten steel is achieved, thereby forming a colored surface; and (ii) sealing at least a portion of the colored surface from stage (3) of step (i) by applying a colorless, transparent coating, thereby forming a surface-treated Corten steel.
 2. The method according to claim 1, wherein the Corten steel comprises: iron as the main component, and one or more components selected from the group consisting of carbon in a proportion of 0 to 0.16% by weight, chromium in a proportion of 0.40 to 0.65% by weight, silicon in a proportion of 0.30 to 0.50% by weight, manganese in a proportion of 0.80 to 1.25% by weight, phosphorus in a proportion of 0 to 0.030% by weight, sulfur in a proportion of 0 to 0.030% by weight, copper in a proportion of 0.10 to 0.40% by weight, vanadium in a proportion of 0.02 to 0.10% by weight, and nickel in a proportion of 0 to 0.40% by weight.
 3. The method according to claim 1, wherein the cleaning in stage (1) of step (i) takes place in such a way that the cleaned surface of the Corten steel is metallically clean, free of grease, and free of rust.
 4. The method according to claim 1, wherein the aqueous activating solution contains fluoride ions in a quantity of greater than 0% by weight and less than or equal to 3% by weight, and/or the aqueous activating solution contains the oxidizing agent(s) in a quantity of greater than 0% by weight and less than or equal to 20% by weight.
 5. The method according to claim 1, wherein the treatment of the activated surface in stage (3) of step (i) is carried out with water over a period of between 30 minutes and 15 hours, up to the desired color of the surface.
 6. The method according to claim 1, further comprising rinsing the active surface with water in stage (2) from step (i), and wherein the coloring of the activated surface in stage (3) of step (i) is carried out by spraying and the activated surface is kept moist.
 7. The method according to claim 1, wherein the sealing in step (ii) takes place using a sol-gel coating, the sol-gel coating being burned in after the application in order to achieve a glass ceramic structure.
 8. The method according to claim 1, wherein at least a portion of the quantity of fluoride ions originates from hydrogen fluoride which has been added to the aqueous activating solution.
 9. A Corten steel having a colorless, transparent coating on a colored surface, with the exception of rust brown, of the Corten steel, wherein the surface has been treated by a method comprising: (i) carrying out a treatment comprising the following stages: (1) cleaning at least a portion of at least one surface of the Corten steel, thereby forming a cleaned surface; (2) activating at least a portion of the cleaned surface of the Corten steel from stage (1) by bringing the cleaned surface into contact with an aqueous activating solution containing fluoride ions, one or more oxidizing agents, and optionally one or more stabilizers for the oxidizing agent(s), thereby forming an activated surface; and (3) coloring at least a portion of the activated surface of the Corten steel from stage (2) by treating the activated surface with water for a period of at least 30 minutes and until the desired coloration of the surface of the Corten steel is achieved, thereby forming a colored surface; and (ii) sealing at least a portion of the colored surface from stage (3) of step (i) by applying a colorless, transparent coating, thereby forming a surface-treated Corten steel, and wherein the sealing step (ii) takes place using a sol-gel coating, the sol-gel coating being burned in after the application in order to achieve a glass ceramic coating.
 10. The method according to claim 2, wherein the cleaning in stage (1) of step (i) takes place in such a way that the cleaned surface of the Corten steel is metallically clean, free of grease, and free of rust. 